The primary monitoring (without cooperation on the part of the aircraft-targets) of air traffic is conventionally done by means of “monostatic” radars, which transmit radio pulses and receive the reflections of said radio pulses by targets (aircraft) to be detected, the transmitter and the receiver of each radar being co-located. There also exist so-called “multistatic” systems, comprising one or more transmitters of radio signals and a plurality of receivers not co-located with the transmitter or transmitters for receiving the reflections. In all these cases, these are “active” systems, transmitting radio signals specifically intended for the detection of targets.
There also exist so-called passive detection systems, which exploit, in a detection scheme of multistatic type, radioelectric signals available elsewhere (“opportunity signals”), for example radiophonic or television signals. One then speaks of passive coherent location (PCL). Purely passive systems are not appropriate for applications as critical as air traffic monitoring; however, it would be possible—at least in principle—to produce “semi-active” systems, using opportunity transmitters according to a cooperative modality, based on an agreement with the operators of these transmitters.
FIG. 1 illustrates the basic principle of multistatic detection, be it active or passive. A transmitter ER transmits a radioelectric signal SRE which propagates in the air and reaches a receiver RR following two routes:
a direct path, T1, whose length L1 is equal to the distance D between the transmitter and the receiver; and
an indirect path, T2, comprising a reflection by a target to be detected C (here, an aircraft), exhibiting a length L2>L1 which depends on the position of said target.
The signal that followed the direct path (reference signal) and the one that followed the indirect path reach the receiver from different directions; they can therefore be discriminated, for example by means of an array antenna equipped with a beam-forming circuit (digital synthesis of the reception pattern). Their relative propagation delay, Δtp=ΔL/c=(L2−L1)/c (c being the propagation velocity of the radioelectric signals, that is to say the velocity of light) can then be determined by cross-correlation, thereby making it possible to calculate L2 (L1 being assumed known). It is then known that the target is situated on an ellipsoid whose foci are the transmitter and the receiver, defined as the locus of the points, the sum of whose distances from the two foci is equal to L2 (“bistatic distance”). If at least two other transmitter/receiver pairs are available (for example, if there are at least three receivers for a single transmitter, or vice versa, or else two transmitters and two receivers etc.), the target C can be located by intersection between the various ellipsoids. In the case of a target in motion, the reflected signal is frequency shifted through the Doppler effect. The correlation is therefore calculated several times, introducing different frequency shifts between the two signals; the value of frequency shift for which the correlation is a maximum makes it possible to determine a “bistatic velocity” which is the derivative of the bistatic distance L2 with respect to time. The velocity vector of the target can be obtained on the basis of three different “bistatic velocities”.
It is easily understood that, for direct propagation to be possible, the transmitter and the receiver must be “in line of sight” thereby implying that, on account of the curvature of the Earth, their distance may hardly exceed a few tens of kilometers, unless at least one of the two is placed at a high altitude, this not always being possible or desirable. This is not a problem in active multistatic radars, in which the radiation patterns of the transmitters are defined as a function of the detection requirements, but becomes so in the case of passive detection systems applied to air traffic monitoring.
Location of the target is affected by an uncertainty whose value depends on the bandwidth of the radioelectric signals which are used for detection. This limits the choice of the opportunity transmitters that may be used for air traffic monitoring. Indeed, the use of FM radiophonic transmitters (bandwidth of about 20 kHz) leads to uncertainties of the order of 1 km and is therefore unsuitable for the monitoring of civil air traffic (although it is appropriate for other applications, for example for aircraft detection), while television signals (bandwidth of about 10 MHz) make it possible to achieve uncertainties of the order of 20 m, this being satisfactory for civil air traffic monitoring. However, television transmitters transmit beams that are substantially parallel to the ground and exhibit, in a vertical plane, a small angular aperture (of the order of 2° to 4°). This implies that an airplane flying at an altitude of 30,000 feet (about 9144 m) is illuminated only by television transmitters situated more than 300 km away.
This situation is illustrated with the aid of FIG. 2. This figure (which is not to scale) represents a transmitter ER on the surface—spherical—of the Earth, ST. The transmitter transmits a radioelectric signal SRE in the form of a relatively narrow beam, propagating along a mean direction parallel to the tangent plane to the terrestrial surface ST at the level of the transmitter. The hatched line TV represents the flight trajectory of a target consisting of a civil airplane. This line is parallel to the terrestrial surface and remains at a distance H, of the order of 9,000 m, from the latter. It may be seen that the beam SRE intercepts the trajectory TV in a detection zone ZD which is very distant from the transmitter ER. On the other hand the receiver RR, which exhibits a height of a few tens of meters at most with respect to the surface ST, must be much closer to the transmitter ER in order to be able to intercept the beam SRE.
Consequently, the coverage ensured by a group of television transmitters and of receivers forming a passive multistatic radar exhibits a “blind cone” at the high altitudes which are of interest in aerial monitoring. FIGS. 3A and 3B illustrate this effect in the case of a multistatic system comprising a television transmitter Tx and three receivers Rx situated in a radius of about 30 km around the transmitter so as to be “in line of sight” (“LOS”). In FIG. 3A, the region RC1000 represents the coverage at an altitude of 1000 feet (304.8 m): it may be seen that it exhibits an approximately convex shape. In FIG. 3B, on the other hand, the region RC30.000 represents the coverage at an altitude of 30,000 feet (9144 m), which is in the shape of a circular ring; there is no coverage in the central region where the transmitter and the receivers are situated. The gaps of the ring correspond to the directions of alignment between the transmitter and a receiver and for which the radiation patterns of the receivers exhibit zeros so as to reject the direct signals. It is readily understood that it is not practical to carry out full coverage of a territory by using ring-shaped detection regions, all the more so as the location of the television transmitters is not optimized for this application. Moreover, the passive location of a target situated at a distance of several hundred kilometers from the transmitters and receivers would be rendered difficult by the significant attenuation undergone by the signals.
Document WO 03/014764 discloses a method of collaborative coherent location not requiring the receivers to be in line of sight of the transmitter or transmitters. This method uses the detection of predefined sequences inserted into the signals emitted by the transmitters so as to alleviate the absence of reference direct signals. This technique is constraining, since it necessitates a modification of the transmitted signals. Furthermore, the integration of the signals received can only be done over the duration of the predefined sequences, which are generally fairly brief; this implies that the method can only operate under good signal-to-noise ratio conditions.
Document EP 1 972 962 discloses a method of passive and non-cooperative coherent location not requiring the receivers to be in line of sight of the transmitters. This method uses the extraction of distinctive characteristics (“fingerprints”) of the signals received after reflection by the target so as to alleviate the absence of reference direct signals. Such a technique can only operate under restrictive assumptions, notably a high signal-to-noise ratio. Furthermore, it seems better suited to analog modulations than to digital modulations, ever more widespread in opportunity signals.
The Final Report “ADT—Alternative Detection Techniques to Supplement PSR Coverage”, prepared by the company Thales Air Systems for Eurocontrol (European organization for air navigation safety), describes an active multistatic radar system in which the reference signal can be sent from the transmitters to the receivers by means of a data transmission link, for example wired. Such a system consists of dedicated transmitters and receivers, arranged according to a predefined layout and at distances of a few tens of kilometers. The use of opportunity signals is not envisaged.
The article by M. R. Inggs et al. “Commensal radar using separated reference and surveillance channel configuration”, Electronics Letters, Vol. 48, No. 18, 30 Aug. 2012 discloses a bistatic radar for aerial monitoring using opportunity signals comprising an opportunity transmitter and a receiver out of sight of said transmitter.